Voltage responsive apparatus



June 14, 1960 'rsENG w. LIAO ErAL 2,941,143

VOLTAGE RESPONSIVE APPARATUS Filed May 15, 1957 2 Sheets-Sheet 1 [ly15M/QM The/'r Hhrngy.

June 14, 1960 TSENG w, LlAo ETAL 2,941,143

VOLTAGE RESPONSIVE APPARATUS Filed )lay 15, 1957 2 Sheets-Sheet 2 (Q)(C) W A'. l v.'

Tse-13g W. Liao,

VOLTAGE RESPONSIVE APPARATUS Tseng W. Liao, Pittsfield, Mass., andAlfred F. Boice,

Shelbyville, Ind., assignors to General Electric Company, a corporationof New York Filed May 15, 1957, Ser. No. 659,279

'6 Claims. (Cl. S24-'54) This invention relates to voltage responsiveapparatus, and more in particular to an improved means for indica-tingthe presence of recurring electrical transients such as corona dischargein electrical equipment. In the latter respect, it is particularlyuseful in the testing of the dielec- Itric strength of electricalequipment, such as transformers,

motors,'generators and the like.

When a voltage applied across the dielectric material of an electricaldevice is gradually increased, a critical point is reached whereinpartial breakdown of the dielectric material occurs as a result of freeelectrons within the dielectric material reaching such an energy levelthat they free additional Velectrons and thus etect an avalanche ofelectrons. This critical point is commonly referred to asvthe coronaonset or corona starting voltage and is a measure of the dielectricstrength of the insulation employed in the electrical device. tion ofthe corona onset voltage is impractical in electrical equipment as aresult of the usual non-uniform electric fields present and theinhomogeneity of insulation -em-l ployed.

The corona occurs only momentarily during one or both half cycles of thevoltage applied across the dielectric material since it is rapidlyneutralized by the building up of a; space charge in the material. Thecorona results in voltage pulses or damped high frequency oscillationssuper posed upon the applied voltage, and these pulses or oscillationsare therefore an indication` of the presence of corona.

In the past, equipment has been provided for determining the coronaonset voltage of electrical apparatus. Such equipment generallycomprises means for applying a-'variable voltage of known magnitude tothe apparatus, and visual means such as an oscilloscope for indicatingthe presence of pulses or oscillations at the terminals of theapparatus. While such apparatus has provided satisfactory indication ofcorona, it has generally been expensive and complicated and technicalskill has been required to interpret'the information provided by theapparatus.

"It is therefore an object of this invention to provide economicalvoltage responsive means for indicating the presence of recurringelectrical transients such'as pulses or' oscillations. f

' Another object is to provide voltage responsive means for indicatingthe presence of recurring electrical transients such as pulses orldamped oscillations and having a gono-go`type of indication to enablethe use of the voltage responsive means by an unskilled operator.

A still further object of this invention is to provide simplified meansfor detecting the presence ofvcorona in an electrical device, the meansbeing characterized -by providing a go-no-goV indication readilyunderstandable by an unskilled operator.

Briefly stated, in accordance with one aspect of our invention, weprovide a voltage responsive apparatus comprising means `for applying avariable alternating Mathematical calcula-l Patented June 14, 1960 ICC 2voltage of determinable magnitude to the terminals of an electricaldevice. For example, the electrical device may be a transformer which is`being tested to determine the corona onset voltage. Frequency sensitivemeans is also connected -to the terminals of the electrical device,-

and the frequency sensitive means provides an output voltagesubstantially free of components of the frequency of the alternatingvoltage applied to the terminals. A pair of indicating means areconnected to the output of the Afrequency sensitive means and gatingmeans are provided for alternately rendering the indicating meansinoperative during opposite half cycles of the alternating voltage.Amplifying means may also be provided between the frequency sensitivemeans and the indicatingk circuit, amplifier, indicating and gatingmeans that may be employed in the arrangement of Fig. 1,

Fig. 3 is a sequence chart illustrating the response of the circuit ofFig. 2 -to a pulse occuring on only one-half cycle of the voltageapplied to an electrical device tested, and

Fig. 4 is a sequence chart illustrating the response of' the circuit ofFig. 2 to damped oscillations occurring in' lboth half cycles of thevoltage applied to an electrical device being tested.

Referring now to the drawing, and more in particular to Fig. f1, thereinis illustrated a -transformer a primary winding 11 and a secondarywinding 12. The primary winding 11 is adapted to be connected secondaryWinding 12 is connected in series with a resistor 13 and an electricaldevice 14 which is to be tested. The capacitance electrical device 14 isherein represen-ted as a -since in the testing of an electrical devicefor the corona onset voltage, a high voltage is impressed across theinsulation of the device and the device acts in effect as a capacitor.Thus, the electrical device 14 may be any electrical equipment having adielectric material disposed between.

conductive parts adapted to operate at different potentials.

The secondary winding 12 of the transformer 10 may be' tapped in orderto provide a variable voltage across the electrical device 14, althoughit is obvious that the transformer 10 may alternatively be anon-variable step-upy transformer supplied by a variable low Meanssuchas a voltmeter 15 connected portion of the secondary winding 12 voltagesource. across the active may be provided in order to indicate thevoltage applied to the terminals.

of the electrical device 14.

13 preferably has non-linear resistance in order to reduce the danger ofexcess voltage at the terminals of the fre-'vv quency sensitive circuit.A pair of visual `indicators 22- and 23 are connected to the output ofthe amplifier 21,' with visual indicator 22 being connected directly tothe output of the amplifier and visual indicator 23 being cony nected tothe output of the amplifier 2,1 by way of a phase being' 10 having to asourcev of alternating voltage of predetermined frequency, and the Afrequency sensitive circuit 20 is connected across the resistor 13, andthe output of the frequency sensitive cir-y cuit 14 may be connected toan amplifier 21. The resistor ondary winding 12, and having its outputconnected tothe visual indicators 22 and 23. Y

Referring now to Fig. 2, the frequency sensitive c rcuit 20is 'comprisedof a pulse transformer 30 and The primary winding 32 of thev pulseIcapacitor 31. transformer 30 is connected in parallel with the resistor13, the `resistor 13 being adapted to be connected in series:

- with the electrical device 14 across the secondary wind ing of thetransformer 10 of Fig. l. A terminal 33, adapted to be connected to theterminal of the electrical device to be tested may be provided connectedto one end of the resistor 13. A protective gap 34 may also be providedin parallel with resistor 13. The'secondary* winding 35 of the pulsetransformer 30 is serially connected with the capacitor 31 and apotentiometer 36.

The pulse transformer 30` and capacitor 31 are frequency sensitive inthe respect that they attenuate low frequency.' voltagesv and provide anoutput on the arm 37 offthel potentiometery 36 that is substantiallyfree of vpower circuit frequencies.

art may alternatively be provided without departing from the spirit orscope of the invention.

The arm 37 of they potentiometer is connected to the control grid of anamplifying tube 4i), thearnplier tube 40 and associated circuitcomprisingl the amplifier 21, and

the output of the tube 40 is connected to the control grid 41 of a vgastetrode 42. lt will of course be obvious that a more elaborate amplifiermay be vemployed to provide greater sensitivity forv the circuit. Theoutput amplitier 21 is also .connected to .a phase inverter 24 by wayjofa resistance voltage divider 43, the phase inverter comprising a triode44, and the output ofthe phase in verter is connected to the controlgrid 45 of gas tetrode late supply voltage for the gas tetrodes 42 and46 is provided by a transformer 47 having a center tapped the voltageapplied to the terminals of the electrical `de-v vice .being tested.Thus, the primary winding 53 may be connected to a low voltage tap onthe transformer Visual indicating means, such as neon lamps y55 and '6are connected in parallel respectively with the Aresistors 49 and 51.vIt will be obvious, of course, that Vother indicating means mayalternatively be employed andarranged to be energized by the platecurrent flow of the gas tetrodes.

- A power supply 57 of any conventional type may 'be employed tovprovide plate voltage for the amplifier 21 and phase inverter 24 andbias supply foi' the gas tetrodes 4 2 and 46. y

In testing an electrical device such as the device 14 of Fig. l, thealternating voltage lapplied tothe terminals of the ldevice by thetransformer is gradually increased. The frequency of the applied voltage`ispreferably low, such as, for example, the power frequency 60v cyclesper second. When the applied voltage reaches the corona onset voltage ofthe device 14, a voltage pulse or damped oscillation vappears across theterminals ofthe device, and this pulse or oscillation is superimposed onthe low frequency voltage appearing across the non-.linear resistor 13in series with the electrical device .14.

If, for example, the corona occurs only asv a pulse on the positive halfcycles of applied voltage, the voltage waveform developed across theresistor 13 may appear asin Fig. 3'(a), which illustrates a pulsesuperimposed on the positivek peak of a single cycle .of the appliedvoltage. The frequency sensitive circuit 20 attenuates It will beobvious, of course, that otherv frequency sensitive means such ras arewell known in the alcalines f1' 4 thelow frequency component of thewaveform of Fig. 3(a), and the voltage input to the amplifier has awaveform comprising a single pulse with substantially no low frequencycomponent, Fig. 3(6). The amplified output of the amplifier, comprisingalso an amplified single pulse recurring during each full cycle of thelow frequency voltage, Fig. 3(0) appears at the control grid of gastetrode 42. The output of the amplifier is inverted by phase inverterZ4, and the inverted.l output voltage waveform, Fig. 3(d), appears atthecontrol grid of gas tetrode 46. The primary winding 53 of thetransformer is connected to the source of applied voltage (eg.

' l to a low voltage tap on the transformer 10 of Fig. l),

` conducts during the second half of applied voltage.

so that the alternating voltage applied to the plate of the gas tetrode42 Fig. 3(e), is 180 out of phase with the applied voltage and thealternating voltage of gas tetrode 46, Fig. 3( f) is in phase with theapplied voltage.

The fixed cathode bias ofthe gas tetrod'es (provided by a resistancevoltage divider lcomprised of resistors 60 and 61) is suicient to holdthe tubes nonconducting until triggered by a positive pulse oroscillation is ap-4 v plied to their control grid. Y

In the preceding example,y no plate current will flow in gas tetrode 42,Fig. 3(`g`), since the plate voltage is negative during the positivehalf cycle of applied voltage,

while gas tetrode 46 will be triggered to provide a plate- Neither tubel The resultant plate current of gas tetrode 46 develops avoltagecurrent, Fig. 3(h), during this half cycle.

across resistor 5=1 and neon tube 56, causing the neon tube 56 to glow.Y

If the corona occurs on both half` cycles of the ap-v plied voltage, forexample as damped oscillations, the

voltage appearing vacross the resistor 13 will be the lowA frequencyapplied voltage having damped oscillations on both positive negativepeaks Fig. 4(a). In this event Y the output of the frequency sensitivecircuit 20, Fig.

4(b), as amplified, Fig. 4(0) is comprised of two sg nals. during asingle cycle of the low frequency applied voltage, the low frequencycomponent being attenuated to be substantially eliminated. rl`hissignal, Fig.v 4(c) "plied voltage, and tube 42 will be triggeredto'providel a plate current, Fig. 4(g), during the negative half cycle'As a result of the plate currents;

is applied to the grid of tube 42, while the inverter Y signal, Fig.`4(d), is applied tothe control grid `of tube- 4.6. VThe plate voltage oftube .42, Fig. 4(e), and tube 46, Fig. 4(f), isthe same as in thepreceding example, and thus ytube 46 will be triggered to provide aplate current, Fig.4(h), during the positive half cycle of apof appliedvoltage. owing in both gas tetrodes, visual indication will vbe providedby both neon'tubes.

From the preceding example, rit is seen that visual indication isprovided by neon tube 56 if corona occursonly on positive half cycles ofapplied voltage, and by both -the neon tubes and 56 if thecoronatoccursl during both half cycles of applied voltage.

" obvious that corona occurring only during negative half cycles ofapplied voltage results in visual indication only by neon tube `55.

The sensivity of the circuit of Fig. 2 may be changed b y various means,such as by adjusting the potentiometer 1. 37, so that the visualindication of the neon tubes occurs voltage applied to theterminals ofthe electrical device,

note the voltage applied to the'terminals when the neonv tubes firstbegin to glow, and if desired, to note which tube glows if they are notboth energized'.

It will be'understood, of course, that, while the form' of the,invention 4herein shown anddescribed constitutes the preferredembodiment of the invention, it is not inV It will beA tended herein toillustrate all of the possible equivalent forms or ramificationsthereof. If will also be understood that the words employed are words ofdescription rather than of limitation and -that various changes may bemade without departing from the spirit or scope of 'the invention hereindisclosed, and it is aimed in the appended claims to cover all suchchanges as fall within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. Voltage responsive apparatus for indicating the presence of randomrecurring electrical transients superimposed upon an alternating voltagecomprising frequency sensitive means for attenua-ting voltages of thefrequency of said alternating voltage, irst and second indicating meansconnected directly and inversely respectively to the output of saidfrequency sensitive means, and gating means synchronized with saidalternating voltage to alternately render said indicating meansinoperative during opposite half cycles of said alternating voltages.

2. Voltage responsive apparatus for indicating the presence of randomrecurring electrical transient voltages superimposed upon an alternatingvoltage comprising frequency sensitive means for attenuating voltages ofthe frequency of said alternating voltage, iirst and second indicatingmeans connected directly and inversely respectively to the output ofsaid frequency sensitive means and arranged to give a visual indicationof said random transient voltages when they have a predetermined minimumamplitude, and gating means connected to said indicating means, saidgating means being synchronized with said alternating voltage to preventindication by said tirst indicating means during positive half cycles ofsaid alternating voltage and to prevent indication by said secondindicating means during negative half cycles of said alternatingvoltage.

3. Corona responsive apparatus comprising means for applying a variablealternating voltage of determinable magnitude to the terminals of anelectrical device, frequency sensitive means connected to said terminalsproviding an output from which voltages of the frequency of saidalternating voltage have been substantially eliminated, first and secondindicating means connected directly and inversely respectively to theoutput of said frequency sensitive means, and gating means alternatelyrendering said indicating means inoperative during opposite ha'lf cyclesof said alternating voltage.

4. Corona responsive apparatus comprising means for applying a variablealternating voltage of determinable magnitude to the terminals of anelectrical device, frequency sensitive means connected to the terminalsof said electrical device, said frequency sensitive means providing anoutput signal substantially free of frequency components of saidalternating voltage, rst and Second 'indicating means energized directlyand inversely respectively by said output signal to indicate thepresence of electrical transients of a predetermined minimum amplitudeat the terminals of said electrical device, and gating meanssynchronized with said alternating voltage and connected to alternatelyprevent indication by said indicating means during opposite half cyclesof said alternating voltage.

5. Apparatus for determining the corona onset voltage of an electricaldevice comprising means for applying a variable alternating voltage ofdeterminable magnitude to terminals of said electrical device,yfrequency sensitive means connected to said terminals for providing anoutput signal substantially free of frequency components of saidalternating voltage, a pair of gas tubes having alternating plate supplyvoltage of opposite polarity synchronized with said alternating voltage,means applying said output signal directly to trigger one of said gastubes and inversely to trigger the other of said gas tubes, and separateindicating means energized by plate current ow of said gas tubes.

6. The apparatus of claim 5 wherein the indicating means comprise neontubes connected in parallel with series resistors in the plate circuitsof said gas tubes.

References Cited in the tile of this patent UNITED STATES PATENTS2,617,858 Brink Nov. 11, 1952 2,691,158 Wittenberg Oct. 5, 19542,703,380 yFrazer Mar. l, 1955 2,802,180 Nye Aug. 6, 1957 2,822,518Jordan Feb. 4, 1958

